Chromosomes dynamically control gene replication or transcription by changing their higher-order structures through methylation modification of their constituent DNA and various modifications (acetylation, methylation, phosphorylation, ubiquitination, etc.) of histones (histones H2A, H2B, H3, and H4).
In general, trimethylation of lysine at the fourth position counted from the N-terminus of histone H3 (H3K4me3) functions to activate transcription, whereas trimethylation of lysine at the 27th position (H3K27me3) functions to suppress transcription. The former and latter modifications are performed by a trithorax complex and Polycomb repressive complex 2 (PRC2), respectively (Non Patent Literature 1 and 2).
The Polycomb gene group was identified as a gene controlling the embryogenesis of Drosophila and is also conserved in vertebrates (Non Patent Literature 3). In Drosophila, the enhancer of zeste protein is a catalytic subunit responsible for the H3K27 methylation modification of PRC2. Both EZH1 (enhancer of zeste homolog 1 (Drosophila)) and EZH2 (enhancer of zeste homolog 2 (Drosophila)) are mammalian homologs of the Drosophila enhancer of zeste (Non Patent Literature 4 and 5). The enzyme activity domains (SET domains) of EZH1 and EZH2 have high homology. In humans or mice, two types of PRC2 exist (PRC2-EZH1 and PRC2-EZH2) which contain EZH1 or EZH2 as a catalytic unit (Non Patent Literature 6 and 7).
In ES cells, EZH1 and EZH2 function cooperatively to participate in maintenance of ES cells (Non Patent Literature 6). EZH1 and EZH2 also act cooperatively on the formation and maintenance of hair follicles and the differentiation of Merkel cells, and both have been reported to also play an important role in maintaining hematopoietic stem cells (Non Patent Literature 8 to 12).
Overexpression of EZH2 has to date been reported in many cancers including prostate cancer, breast cancer, stomach cancer, lung cancer, ovarian cancer, pancreatic cancer, kidney cancer, and head and neck cancer, and the poor prognosis in some of these cancers reportedly correlates with overexpression of EZH2 (Non Patent Literature 13 to 21). There are reports stating that EZH2 knockdown of a cell line derived from such a cancer inhibits cell growth (Non Patent Literature 13 and 22). When EZH2 is overexpressed in an epithelial non-cancer cell line, phenotypes characteristic of cancers appear, such as invasiveness and increased cell growth in a soft agar medium (Non Patent Literature 14).
In follicular lymphoma or follicular center B cell-type diffuse large B-cell lymphoma, somatic mutations have been found in tyrosine 641, alanine 677, and alanine 687 (Y641F, Y641N, Y641S, Y641H, Y641C, A677G, and A687V) of EZH2, and these mutations have been reported to render EZH2 hyperactive to significantly increase intracellular H3K27me3 modification levels (Non Patent Literature 23 to 26). Compounds specifically inhibiting the enzyme activity of EZH2 inhibit, both in vitro and in vivo (xenograft models), the growth of a cancer cell line having such a somatic mutation in EZH2 (Non Patent Literature 27 and 28).
These facts suggest that knockdown of EZH2 or inhibition of its enzyme activity is useful in the treatment of cancers involving overexpression of EZH2 or a somatic mutation in EZH2.
Although much is known about the malignant transformation of cells containing EZH2, much remains to be analyzed on the relation of EZH1 to the malignant transformation of cells. However, it has recently been found that general inhibition of PRC2 suppresses the progression of acute myeloid leukemia caused by MLL-AF9 fusion gene, whereas inhibition of EZH2 alone is not sufficient for this suppression (Non Patent Literature 29). This means that inhibition of PRC2-EZH2 alone is insufficient for suppressing acute myeloid leukemia caused by MLL-AF9 fusion gene, and that simultaneous inhibition of PRC2-EZH1 and PRC2-EZH2 is necessary for this purpose.